This proposal is founded upon: 1) the recognition that bone density acquired early in life is a predictor of osteoporosis risk, 2) recognition that peak bone density in women is achieved during the second decade of life, and 3) our current observation that 19 year old women who have been menstrually irregular during their teenage years have significantly lower trabecular bone density than their eumenorrheic classmates. Additional findings which support the concept that maturational events during puberty can affect women's health status decades later include: a documented decrease in adolescent calcium intake, a decrease in adolescent exercise, and linear premenopausa bone loss. Since premenopausal bone loss appears to be influenced by sex steroid status, diet and exercise, we propose a longitudinal study to determine the roles played during adolescence by integrated estrogen status, calcium intake, and exercise upon the timing and magnitude of peak bone density accretion in adolescent women. We propose a five year study of 100 closely matched Caucasian school girls who will be 11 years of age at entry. Fifty of the study group will be given a 600 mg per day calcium supplement by a double-blind design. Each subject will be seen every six months at which time integral vertebral and cortical radial bone density will be measured. Investigations of determinants of bone density in young women have been limited due to the radiation exposure associated with previously available instrumentation necessary to obtain quantitative measurement of ben density. A recent advance in bone measurement technology, x-ray absorptiometry (XRA), in now available and is well suited for studies involving children because of the extremely low radiation used. Nutrition profiles will be obtained from three day diet diaries and exercise patterns from activity surveys. We will measure integrated estrogen exposure by evaluating pubertal progression based on developmental landmarks in addition to sex steroid and gonadotropin profiles obtained from urine specimens. Statistical analysis of the continuous study variables will allow us to determine whether these variables affect peak bone accretion in young girls independently or in a concerted fashion. The results of this study will provide information to be used in the prevention of osteoporosis in American women.